[asterisk-dev] 'prematuremedia = yes' causes odd behavior on SNOM360 with early media.

[David Vossel]

----- Original Message -----
> From: "Jonathan Rose" <jrose at digium.com>
> To: asterisk-dev at lists.digium.com
> Sent: Friday, July 1, 2011 3:55:38 PM
> Subject: [asterisk-dev] 'prematuremedia = yes' causes odd behavior on SNOM360 with early media.
> I was tracking an issue reported by Steve Pitts against an issue which
> involves chan_sip's premature media filter which was added in a commit
> by oej some time ago, and I was looking for an opinion... here's the
> scenario.
> 
> User calls in to Asterisk from basically any SIP phone.
> 
> Phone to Asterisk: INVITE
> Asterisk to Phone: 401 Unauthorized
> Phone to Asterisk: ACK
> Phone to Asterisk: INVITE (with credentials)
> Asterisk to Phone: 100 Trying
> Phone to Asterisk: ACK
> 
> 
> At this point, the call is connected and depending on the codec used,
> it
> will hear audio from what I can only assume is earlymedia. The phone
> for the sake of this walk through is a SNOM360. I've also tested this
> with Blink softphone. The behavior at the phone end is different, but
> the SIP messages are essentially the same.
> 
> Once the call is connected, a 183 session progress is scheduled to be
> sent to the phone once per minute due to another patch oej was
> involved
> with.
> 
> 
> This next bit might be a little unrelated, but around 42 seconds into
> the call,
> 
> At around 42 seconds, the following small conversation:
> 
> Asterisk to Phone: OPTIONS
> Phone to Asterisk: 200 OK
> 
> The only significance I can think of that is that the SNOM phone is
> sending a 200 OK... but again, I don't really believe this part was
> important.
> 
> Finally, at 60 seconds the prescheduled 183 Session Progress is sent,
> and this causes different behavior depending on the receiving device.
> In the case of the SNOM360, the phone will start generating a ring and
> will continue to do so until the call is answered. In the case of
> Blink, nothing immediately special will happen, but a minute or two
> later (I haven't timed it), Blink will give up on the call and say it
> failed to connect.
> 
> If I set sip.conf to have 'prematuremedia=no', then instead of
> Asterisk
> sending a 100 trying and then getting an ACK, Asterisk sends a 100
> trying and a 183 Session Progress which according to what I've read,
> indicates immediately that we are in early media mode and everything
> works as expected.
> 
> So anyway, I have read over some commit logs, and I'm aware that there
> are good reasons for having prematuremedia mode act the way it does
> and
> for it to be set by default. I talked with David Vossel, and he seems
> to agree with my current leanings... that it isn't appropriate to send
> 183 60 seconds later if we didn't send it at the start of the call. At
> the same time, I think we still need to warn the connected device that
> there is a need to keep the connection alive, and so I wanted to ask
> asterisk-dev if there was anyone who knew what, if anything should be
> sent in this situation.
> 
> ~Jonathan Rose, still fairly newish Asterisk developer.
> 

I think this is a result of the provisional keep alive stuff we do.  If the last provisional message was 100, I'd expect 100 to be used for the keep alive 60 seconds later as well.

~David